In an ideal club design, for a constant total swing weight, the amount of structural mass would be minimized (without sacrificing resiliency) to provide a designer with additional discretionary mass to specifically place in an effort to customize club performance. In general, the total of all club head mass is the sum of the total amount of structural mass and the total amount of discretionary mass. Structural mass generally refers to the mass of the materials that are required to provide the club head with the structural resilience needed to withstand repeated impacts. Structural mass is highly design-dependent, and provides a designer with a relatively low amount of control over specific mass distribution. Conversely, discretionary mass is any additional mass (beyond the minimum structural requirements) that may be added to the club head design for the sole purpose of customizing the performance and/or forgiveness of the club.
In many club head designs, discretionary weight may be affixed to the body of the club head by screwing the weight into a threaded port provided on the club head. If a user elects not to utilize additional weight, or to utilize fewer than all available threaded ports, then the unused ports are typically left open where they are susceptible to being impacted with dirt or debris.